1. Field of the Invention
Embodiments of the invention relate to a liquid crystal display and a local dimming control method thereof.
2. Discussion of the Related Art
Liquid crystal displays have been widely used in personal computers such as notebook PCs, office automation equipments, audio/video equipments, interior/outdoor advertising display devices, and the like. The liquid crystal displays display an image using a thin film transistor (TFT) as a switching element. A backlit liquid crystal display occupying most of the liquid crystal displays controls an electric field applied to a liquid crystal layer and modulates light coming from a backlight unit, thereby displaying an image.
The image quality of the liquid crystal display depends on its contrast characteristic. Improvements to the contrast characteristic are limited using only a method for controlling a data voltage applied to the liquid crystal layer to modulate a light transmittance of the liquid crystal layer. As a solution, a backlight dimming method has been proposed so as to improve the contrast characteristic. The backlight dimming method adjusts a luminance of a backlight unit depending on an image displayed on the liquid crystal display. The backlight dimming method includes a global dimming method for adjusting a luminance of the entire display surface of the liquid crystal display and a local dimming method for locally controlling a luminance of the display surface of the liquid crystal display. The global dimming method can improve a dynamic contrast ratio measured between two adjacent frames. The local dimming method can locally control the luminance of the display surface of the liquid crystal display within one frame, thereby improving a static contrast ratio which is difficult to improve using the global dimming method.
A liquid crystal display capable of selectively implementing a two-dimensional (2D) image and a three-dimensional (3D) image has been recently developed based on the development of various image processing techniques. The application of the local dimming method has been considered to implement the 3D image, so as to improve the contrast ratio and reduce the power consumption.
In a related art local dimming technique, the backlight unit is divided into a plurality of blocks. The local dimming technique adjusts a dimming value of each of the plurality of blocks, thereby increasing a luminance of the backlight unit belonging to the block displaying a bright image and reducing a luminance of the backlight unit belonging to the block displaying a dark image. A modulation of pixel data may compensate for a deficiency of the luminance of the backlight unit resulting from the local dimming technique. The modulation of the pixel data is performed based on a pixel gain value obtained through the result of an analysis of an amount of light of the backlight unit of each block.
FIG. 1 illustrates a compensation for pixel data in the related art local dimming technique. As shown in FIG. 1, a dimming value of each block in an Nth frame is output in a vertical blank period VB between the Nth frame and an (N+1)th frame, where N is a positive integer. A pixel gain value of the Nth frame is obtained based on the dimming value of each block of the Nth frame, and then data to be displayed in the (N+1)th frame is compensated using the pixel gain value of the Nth frame. In FIG. 1, Vsync denotes a vertical sync signal, and DE denotes a data enable.
The related art local dimming technique is applied to the implementation of the 2D image without difficulty. However, when the related art local dimming technique is applied to the implementation of the 3D image, the following problems are generated.
There are known several methods for implementing the 3D image. Out of the several methods, a liquid crystal shutter glasses method alternately displays left eye images and right eye images on a display element in units of frame and opens or closes left and right eye shutters of liquid crystal shutter glasses in synchronization with a display timing, thereby implementing the 3D image. The liquid crystal shutter glasses open only the left eye shutter during the frames which display the left eye images and open only the right eye shutter during the frames in which display the right eye images, thereby making binocular disparity in a time divisional manner. In particular, a liquid crystal display using the liquid crystal shutter glasses method has been recently driven in a method (called ‘LBRB’ method), in which a black frame is inserted between adjacent left and right eye data frames, so as to solve an image tailing phenomenon resulting from a response characteristic delay of liquid crystals. In the ‘LBRB’ method, ‘L’ indicates the left eye data frame, ‘R’ indicates the right eye data frame, and ‘B’ denotes the black frame.
However, as shown in FIG. 2, when the related art local dimming technique is applied to the implementation of the 3D image, a gray level saturation is generated in data to be displayed in the left or right eye data frame L/R. This is because a pixel gain value calculated in the black frame B, which is a frame just prior to the left or right eye data frame L/R, is applied to the data to be displayed in the left or right eye data frame L/R. The pixel gain value is determined based on data required to allow a luminance obtained from a total amount of light (i.e., an amount of light in dimming) reaching a corresponding pixel in the local dimming using a dimming value of a block including the corresponding pixel to be equal to a luminance obtained from a total amount of light (i.e., an amount of light in non-dimming) reaching the corresponding pixel in local non-dimming. The pixel gain value is calculated by dividing the amount of light in the non-dimming having a fixed value by the amount of light in the dimming. Therefore, when the amount of light in the dimming is less than the amount of light in the non-dimming, the pixel gain value of the corresponding pixel increases. The pixel gain value calculated in the black frame B increases because of the small amount of light in the dimming. Because an upward modulation width of data increases as the pixel gain value increases, the gray level saturation, is generated in a high gray level section. In other words, in the related art local dimming technique applied to the implementation of the 3D image in the same manner as the implementation of the 2D image, a pixel gain value of a current frame is calculated based on a dimming value obtained in the current frame, and the pixel gain value is used to compensate for data of the next frame. Therefore, the pixel gain value applied to the next frame is not proper in the 3D mode having a little correlation between the adjacent frames (for example, the frames B and L or the frames B and R). The gray level saturation increases a 3D crosstalk and reduces the display quality.